In recent years, with a requirement to increase a travel distance per charge for wide use of an electric vehicle (xEV), higher energy density has been strongly requested for a lithium-ion secondary cell which is a power source of the electric vehicle, from a viewpoint of weight reduction.
One means for achieving the higher energy density is to increase capacity of a cell. There is a method of using a solid solution positive electrode material having Li2MnO3 as a mother structure in a positive electrode, and an alloy mainly including silicon or an oxide thereof as a negative electrode material in a negative electrode (PTL 1).
Silicon indicates a theoretical capacity (4200 mAh/g) far higher than a theoretical capacity (372 mAh/g) of a carbon material currently in practical use, but has not yet been in practical use because a great volume change caused by charging and discharging leads to cell deterioration.
Furthermore, it is described that, in a lithium-ion secondary cell according to PTL 2, a depressed portion 17 is provided on a surface of a negative electrode active material layer on a side contacting a separator, and it is thereby possible to prevent the entire cell from expanding due to expansion of the negative electrode active material layer at a time of charging.
Still further, in a secondary cell according to PTL 3, a large number of stress relaxation spaces, specifically, depressed grooves 133m are formed in a negative electrode active material layer 133 in order to relax stress generated in the negative electrode active material layer 133 due to expansion and contraction at times of charging and discharging. The depressed grooves 133m are pierced between a separator 141 and a negative electrode plate 131.
Yet further, it is described that, in a lithium-ion secondary cell according to PTL 4, progress of a crack and electrode breakdown resulting from charging and discharging can be inhibited by providing a groove in the vicinity of an outer periphery of a main surface of an electrode active material sintered compact.